W4: Gravity wave parameterisation for the ocean

Principal investigators: Prof. Carsten Eden (Universität Hamburg), Dr. Johann Jungclaus (Max Planck Institute for Meteorology), Prof. Dirk Olbers (MARUM/AWI)

Space-time scales of important oceanic processes (pink areas) and scales explicitly resolved by ocean models (grey rectangular areas). The lower left rectangle represents modern global ocean climate models and the upper right rectangle eddy resolving basin-scale models. Also shown are dispersion curves (solid lines) for linear gravity waves (upper set) and planetary waves (lower set). Vertical dotted lines indicate the external (Ro) and the first internal (Ri) Rossby radii and the Ozmidov length scale Lo.

The recently proposed parameterization module "Internal wave Dissipation Energy and MIXing" (IDEMIX) describes the generation, propagation, interaction, and dissipation of the internal gravity wave field and can be used in ocean general circulation models to account for vertical mixing (and friction) in the interior of the ocean. It is based on the radiative transfer equation of a weakly interacting internal wave field, for which spectrally integrated energy compartments are used as prognostic model variables. IDEMIX is central to the concept of an energetically consistent ocean model, since it enables to link all sources and sinks of internal wave energy and furthermore all parameterized forms of energy in an ocean model without spurious sources and sinks of energy.

An improved IDEMIX model for the ocean will be constructed in W4, extended by a new highfrequency, high vertical wavenumber compartment, forcing by mesoscale eddy dissipation, anisotropic tidal forcing, and wave-mean flow interaction. All these processes have never been implemented in ocean models but have an important effect on mixing and the energy transfers in the ocean. We will validate the simple and more complex versions of IDEMIX and the new version using available fine- and  microstructure datasets. The simple and more complex IDEMIX versions will be implemented into the ICON and FESOM ocean models.

Validation of the simple IDEMIX version: a) Observational estimate of the dissipation of gravity wave energy calculated from density profiles of ARGO floats following Whalen et al. (2012) averaged between 500 and 1000 m. b) Same as a) but simulated by IDEMIX. Taken from Pollmann, Eden and Olbers (2016, in review at JPO).
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How the background mean flow effects internal gravity waves

From my work, hopefully general rules may be seen that can be included in parameterisations for internal gravity waves.

Rachael Ewins, PhD in W4

I am investigating the effect background mean flow has on the propagation of internal gravity waves. From this hopefully general rules may be seen that can be included in parameterisations for internal gravity waves. For this ray tracing is used to follow the positions and properties of wave packets that interact with an idealised current.

The test wave packets are populated randomly over a range of physical positions and also phase space, which allows exploration of the importance to various properties to how the test wave packets interact with the background current. The key property that is being tracked is the energy of the packets and from this the transfer of energy to and from the current can be seen.

Ray tracing simply propagates the position and wave numbers of the wave packets over a series of time steps given that background properties of background flow velocity, the local buoyancy frequency. The energy of the wave packets can be followed due to the conservation of Action. The results means that individual wave packets can be followed to different end conditions namely critical layer absorption, wave capture or refraction away from the current flow. The net energy transfer from the waves to the background flow (or from) can be seen by the end energy of the waves that enter critical layers or are captured by the current.

By varying the properties of the background current the effects of various shears in the current can be seen which will lead to more information about the key properties of both internal wave and background flow that lead to wave captures and critical layer absorption. In addition the background flow can be changed into configuration to simulate eddies, using the same processes.